Atomic-scale investigation of deformation mechanisms in Mg alloys

Mg alloys have excellent strength to weight ratio but their use is limited by their poor room temperature formability. The underlying hexagonal closed pack structure results in a highly anisotropic deformation; basal deformation is easily activated, whereas high stresses are required to accommodate the deformation along axis. We study the atomic-scale mechanisms of Mg nonbasal deformation modes that involve slip and twinning. Our study includes two aspects: accurate modeling of defect structures and interactions with solutes and investigation of solute strengthening. The core structures of edge and screw dislocations in Mg are computed using density functional theory (DFT). Both types dissociate into two ½ partials on the second-order pyramidal planes. We show that earlier core structures based on embedded-atom method are artifacts of the interatomic potentials and are not accurate. The DFT core structures are used for further investigation of solute effects. In addition, solute strengthening of twin dislocation motion along an existing twin boundary in Mg-X (X = Al, Zn) is investigated using a new Labusch-type weak pinning model. New features emerge in the application of the model because of the very small Burgers vector of the twin dislocation. The strengthening is not large, e.g., a strength of ≈10 MPa is predicted for the AZ31 alloy, but the analysis does predict larger strengthening of twinning compared with basal slip at room temperature and various concentrations. The predictions are compared with existing experimental data and are shown to agree well with the experiments. The methods discussed here, can be applied to a wide range of defect calculations with chemistry change, guiding towards a well-informed design of materials on an accurate foundation

A first principles study of defects in titanium: interaction of twin boundaries with dislocations and oxygen interstitials

Interaction between gliding dislocations and twin boundaries affects the plastic deformation of hcp metals such as titanium. In addition, oxygen greatly affects both strength and twinning in titanium. Predictive models of strength and twinning rely on understanding of the underlying atomic scale mechanisms which are best captured through computer simulations. While recent first-principles methods predict dislocation core structures and boundary geometries and energies, modeling a dislocation near a boundary requires new techniques to treat the long range strain field of the dislocation near a boundary. Using flexible boundary conditions with a new method to compute the lattice Green’s function for crystals containing a planar interface, we present a general method to study line defects interacting with interfaces with a tractable number of atoms. This method is general in the sense that it can consider long range atomic interactions and reconstructions near the interface. We use the interfacial lattice Green’s function to model a screw dislocation interaction with Ti (10-12) twin boundary for the first electronic structure prediction of a dislocation in a boundary. We predict the dislocation core geometry in the twin boundary and compare with the core structure in bulk titanium. The first principles nature of this study makes it possible to consider interactions with solutes. The interaction energy of an oxygen interstitial with the Ti (10-12) is also computed. While we applied our method to a systematic study of defects interactions in titanium, the method is general and opens up the possibility of investigating line defects/interface interactions with chemistry changes in arbitrary systems

Lattice Green's function for crystals containing a planar interface

Flexible boundary condition methods couple an isolated defect to a harmonically responding medium through the bulk lattice Green's function; in the case of an interface, interfacial lattice Green's functions. We present a method to compute the lattice Green's function for a planar interface with arbitrary atomic interactions suited for the study of line defect/interface interactions. The interface is coupled to two different semi-infinite bulk regions, and the Green's function for interface-interface, bulk-interface and bulk-bulk interactions are computed individually. The elastic bicrystal Green's function and the bulk lattice Green's function give the interaction between bulk regions. We make use of partial Fourier transforms to treat in-plane periodicity. Direct inversion of the force constant matrix in the partial Fourier space provides the interface terms. The general method makes no assumptions about the atomic interactions or crystal orientations. We simulate a screw dislocation interacting with a $(10\bar{1}2)$ twin boundary in Ti using flexible boundary conditions and compare with traditional fixed boundary conditions results. Flexible boundary conditions give the correct core structure with significantly less atoms required to relax by energy minimization. This highlights the applicability of flexible boundary conditions methods to modeling defect/interface interactions by \textit{ab initio} methods

Semantic-Web Architecture for Electronic Discharge Summary Based on OWL 2.0 Standard

ABSTRACT Introduction: Patients’ electronic medical record contains all information related to treatment processes during hospitalization. One of the most important documents in this record is the record summary. In this document, summary of the whole treatment process is presented which is used for subsequent treatments and other issues pertaining to the treatment. Using suitable architecture for this document, apart from the aforementioned points we can use it in other fields such as data mining or decision making based on the cases. Material and Methods: In this study, at first, a model for patient’s medical record summary has been suggested using semantic web-based architecture. Then, based on service-oriented architecture and using Java programming language, a software solution was designed and run in a way to generate medical record summary with this structure and at the end, new uses of this structure was explained. Results: in this study a structure for medical record summaries along with corrective points within semantic web has been offered and a software running within Java along with special ontologies are provided. Discussion and Conclusion: After discussing the project with the experts of medical/health data management and medical informatics as well as clinical experts, it became clear that suggested design for medical record summary apart from covering many issues currently faced in the medical records has also many advantages including its uses in research projects, decision making based on the cases etc